Analyzing the Tribology of High-Entropy Alloys Prepared by Spark Plasma Sintering

Abstract

High-entropy alloys (HEAs) are prospective advanced materials for the production of components that operate at high, severe friction and in high-temperature environments. This is because they possess unique properties requisite for such applications. Hence, this study was aimed at reviewing most recent publications on the tribological characteristics of HEAs processed with spark plasma sintering (SPS). The choice of SPS was because it impacts alloys with a homogenous microstructure, high wear resistance, densely packed grains, and nanocrystalline microstructure. The resource materials for this study were obtained from the Scopus-indexed journal/Google Scholar website for articles published within the last five years. From the study, it was observed that HEAs have good tribological properties which permit their prospective usage in the production of strength-demanding, wear-demanding, and temperature-demanding components. The addition of BCC-forming and FCC-forming elements would help in improving the wear properties of HEAs. It was also observed from the literature that the incorporation of post-processing treatment, laser cladding, shot peening, or the coating of SPSed composites would increase the effective performance and durability of HEAs prepared with SPS.

1. Introduction

High-entropy alloys (HEAs) are typical metallic alloys comprising five elements or more combined in equi-atomic or near-equi-atomic composition. Each element in the alloy possesses an atomic concentration of 5–35%. The entropy of formation of HEAs is very high, and this attribute contributes to their exceptional characteristics. Meanwhile, their enthalpy of formation is generally small in comparison with that of conventional alloys, and this contributes to their metastable structural configuration. Hitherto, scholars were saddled with the material’s challenges of high-temperature oxidation, high-temperature creep, and high-temperature loss of strength experienced in aerospace, automotive, electrical, and structural systems. So, the burning zeal to develop a more robust material via research and innovation which can mitigate those challenges led to the discovery of this all-important alloy. Recall that conventional alloys like Ni, Ti, and Al alloys exhibited the depreciation of strength when used at temperatures between 350 and 650 °C. So, their performances above these temperatures were virtually below average, and this contributed to the failures of some aerospace, automotive, and structural components and subsequent poor durability [1,2,3,4]. So, in 2004, two independently published articles by Yeh and Cantor first made the mention of HEAs. From then, the study of HEAs grew sporadically. Ever since scholars observed that a large number of elements involved in the formation of HEAs stimulated the evolution of solid solution crystal structure, their interest in them tripled. There are so many excellent attributes of HEAs that provoked the curiosity of academia, some of which will be deliberated in this study [5,6].

There exist a number of phenomenal effects that occur during the formation of HEAs which contribute to their unique properties. These effects include high entropy, lattice distortion, sluggish diffusion, and a cocktail. The high-entropy effect is the concept which explains that HEAs experience a high degree of randomness during formation which induces unique properties in them than it is experienced in traditional alloys. Such properties include a single-phase solid solution, a refined microstructure, homogenous and ultra-fine grains, etc. In lattice distortion, it is a concept which establishes that the kinetics of their formation, stimulating the spinning and straining of atoms domiciled at the lattice configuration and subsequently promoting solid solution hardening, grain borderline pinning, and dislocation suppression hardening. The sluggish diffusion effect describes that the HEA’s formation suppresses grain coarsening, reduces phase separation, and enhances the thermal strength and creep stability via very slow inter-atomic diffusions. The cocktail effect is the concept upholding that each constituent element contributes a particular property to the group such that they will synergize to produce some unique properties [7,8,9,10,11]. It is the combination of all these effects and concepts that bestows unique properties upon HEAs.

Ever since the discovery of HEAs by Cantor and Yeh, research in this topic has risen geometrically by each passing year due to the potential applications of the alloy. The number of publications on HEAs as obtained from the Scopus database is displayed in Figure 1.

Here, it can be observed that, from 2013 to 20 October 2023, a total of 8888 peer-reviewed journal articles (Figure 1a) and 396 peer-reviewed review articles on HEAs (Figure 1b) were published in Scopus-indexed journals. This number excludes conference papers and papers published in other indexing journals. This is a confirmation that this topic has gained a lot of attention, even though more work is expected to be performed on them because of their importance and supposed versatility. Techniques used in the fabrication of HEAs are manifold and various researchers have reported on a number of procedures. Techniques reported included solvo-thermal synthesis [12], ultrasonic method [13], carbo-thermal shock method [14], bed pyrolysis [15], etc. However, it was discovered that these techniques were not sustainable in terms of productivity, scalability, energy-saving, and economy.

So, researchers have the embraced spark plasma sintering (SPS) method as among the most robust consolidation processes due to its unique characteristics of grain refinement, homogenous dispersion of dispersed phases, grain boundary interlocking, vaporization of impurities, and enhancement of microstructural, tribological and mechanical properties [16,17,18]. Fu et al. [19] produced the Co_0.5FeNiCrTi_0.5 alloy by mechanical alloying (MA) and SPS. The results showed enhanced micro hardness with refined microstructure. Yeh et al. [8] noticed that the NiCoCrCuFe HEA prepared via spark plasma sintering generated more improved microstructure and strength than that fabricated with stir casting. Moazzen et al. [20] developed Fe_XCoCrNi (x = 1–1.6) alloy using MA and SPS (KCE-FCT-HHPD 25, Germany). It was noticed that, when the Fe fraction was raised, the UTS rose from 480 MPa to 560 MPa, while the hardness rose from 320 HV to 400 HV. Moreover, the coefficient of friction (COF) and wear volume decreased tremendously. Exploits of SPS cannot be over-emphasized, hence the choice of the technique in this research.

SPS is described as a non-conventional sintering technique employed in powder metallurgy which makes use of the pulsed direct current (DC) applied concurrently with axial pressure to consolidate a bulk mass of alloys, compounds, or composites. The bulk mass usually possesses an enhanced microstructure without pores, zero grain coarsening, inter-locked grain borderlines, cohesive matrix/additive interface, and uniformly dispersed reinforcements [21,22,23]. Figure 2 shows the fundamental operation of an SPS machine. It was reported that SPS achieves pure products by the action of the pulsed DC current which produces a very high temperature increase within the powder mass in such a way that all impurities are vaporized. The swift heating and cooling rates are responsible for the suppression of grain growth and unwanted intermetallic compounds. The simultaneous application of heat and di-axial pressure is responsible for the generation of highly densified bulk mass.

Meanwhile, it can be observed from Figure 1 that review articles on HEAs are not as sufficient as it is supposed to be considering the significance of HEAs to the global energy security, whilst bearing in mind the energy crisis in terms of cost, sustainability, and carbon emission-free energy sources. It is very unfortunate that the SPS technique lacks the adequate research it deserves that will fast-track its scalability, affordability, and energy conservation abilities. So, the need for in-depth and versatile research into these two challenges (HEA material and SPS method) informed the choice of this study. Hence, the aim of this study is to review the latest works on SPSed HEAs in order to project some milestones that have been covered and ascertain areas that are yet to be touched. The tribological properties of HEAs is the central theme of this study.

Table 1. Characteristics of levels of wear mechanism [24].

Wear Level	Wear Manifestation	Debris Type	Wear Coefficient	Friction Coefficient
Low wear	Superficial deformation	None manifested	10−8–10−6	μ < 0.01
Mild wear	Superficial minicracks and fracture at asperity level	Powder-like debris	10−4–10−2	0.01 < μ < 0.5
Severe wear	Sub-surface cracks and macro fracture	Flake-like debris	10−2–1	μ > 0.5

shows the characteristics of wear mechanisms—while a brief summary of the mechanisms of wear is presented pictorially in Figure 3. Here, we have about five modes of wear that ravage materials. The full detail of the wear mechanisms is presented in Section 4.

Meanwhile, let us look into how HEAs stand out when other alloys are brought forward. In

Table 2. Advantages of high-entropy alloys over traditional alloys.

High-Entropy Alloys	Traditional Alloys	Refs.
Their high-entropy effect induces more unique properties like better tribology	There is no high-entropy effect during the formation of traditional alloys.	[25]
Their high lattice distortion promotes solid solution strengthening and blocking of dislocation slip	Lower lattice distortion reduces solid solution strengthening with higher dislocation slip	[26,27]
The sluggish diffusion in HEAs improves thermal protection and creep stability	Diffusion of atoms is higher in traditional alloys and they experience higher creep	[25,28,29]
The cocktail effects synergize many different properties into one superior entity	Fewer elements are involved and there are no cocktail effects	[7]
Presence of many corrosion products which improves corrosion resistance in HEAs	Corrosion products are not as many as in HEAs	[30]
Higher configurational entropy which increases thermal stability	Configurational entropy is lower in traditional alloys	[31]
Higher structural disorder in HEAs increases dislocation slip energy which enhances their strength and ductility	Structural disorder during formation of traditional alloys is low, so lower strength is obtained	[10]
Their high degree of randomness make them less susceptible to high temperature oxidation	More susceptible to high temperature oxidation	[32]

, the comparative advantages of HEAs over traditional alloys are presented. It can be seen that HEAs are a group of alloys with manifold prospective advantages if fully developed.

Research on the tribological properties of HEAs was motivated by the fact that HEAs are very useful materials with prospective applications in high-temperature, high-pressure, and high-wear-demanding environments. These environments require materials with high strength and high wear resistance to perform efficiently and durably. Hence, an in-depth study on this topic is not only imperative but timely as the global energy crisis escalates. The organization of this work was as follows: Section 1 was the introduction. In Section 2, factors that improve the tribological properties of HEAs were discussed, while in Section 3, the analysis of the wear mechanisms in HEAs was presented. In Section 4, the challenges and future works on the SPS of HEAs were discussed while the conclusion and recommendations are presented in Section 5.

2. Factors That Enhance the Tribological Properties of Spark Plasma Sintered HEAs

Tribological properties of materials encompasses all concerning the material’s behaviors when it is in a relative motion with another contacting body. So, in discussing tribology, concepts like coefficient of friction (COF), wear rate, wear volume, asperity, wear mode, lubrication, etc. are all inclusive. We discussed the factors that enhance the tribological characteristics of HEAs in this section.

2.1. Grain Refinement

HEAs have unique tribological properties such that they are being considered as the most robust and advanced materials for the development of landing gears and brake systems in airplanes, automotive engine pistons, clutches, and brake systems, and for artificial joints (hips and knees) in medical implants [33,34,35]. The uniqueness of their tribological characteristics is attributed to a lot of factors of which the refined/fine-grained microstructure is standing out. Their fine-grained microstructure stimulates the high density of grain boundaries which blocks the dislocation slip so as to improve the strength and tribological properties. Just to increase the grain refinement of the microstructure so as to enhance wear resistance and strengthening mechanism of CoCrFeMnNi, Nagarjuna et al. [36] combined the high-energy mechanical milling and SPS. There was an increment in compressive yield strength from 370 to 1050 MPa, while the wear rate and COF decreased to as little as 1.03 × 10⁻⁵ mm³/Nm and 0.283, respectively. Those enhancements were as a result of the increased high energy milling time that contributed to the refining of the grains (as can be seen in Figure 4), pronounced grain boundary, and blocked-dislocation strengthening provoked by the technique. From Figure 4, it would be seen that increasing the ball milling time transformed the powder morphology from spherical to flattened, and then to flake, followed by an irregular and finally partially spherical shape. Hence, this indicated that the size and morphology of the powder particles contributes enormously to its properties as supported by the Hall–Petch effect [37].

In another study [38], it was observed that strength accruing from even dispersion, pinned grain borders, and refined grains was responsible for the improvement in the strength and hardness of Al_0.2Co_1.5CrFeNi_1.5Ti_0.5C_2.0. Besides those factors, the formation of the oxide layer of TiO₂ in the HEA increased the wear resistance by 40 times that of Al_0.2Co_1.5CrFeNi_1.5Ti_0.5, implying that the refined grains and formation of the oxide layer were instrumental to the wear property’s improvement. The development of equi-atomic Ti₂₀Al₂₀V₂₀Fe₂₀Ni₂₀ HEA was conducted in a study to analyze the influence of heating on the microstructural and tribological properties of the alloy. The evolution of both the body-centered cubic and face-centered cubic in the microstructure together with the grain refinement (Figure 5) was observed. Studying the microstructure before and after sintering with the scanning electron microscope (SEM), it was confirmed that the particle sizes of the powders before sintering (Figure 5a–e) were larger than the grains after sintering (Figure 5f,g). The grain refinement together with the evolution of the body-centered cubic and face-centered cubic phases contributed to the reduction in wear rate to 0.00013 mm³/nm which was 130% lower than that of Inconel 718; and a very low value of COF [39].

2.2. Point Defects

High concentration of point defects is another characteristic of SPSed high-entropy alloys occasioned by the multiple elements with varying atomic sizes that are involved in their formation. Point defects can enhance the wear characteristics of HEAs by decreasing the dislocation motions and reducing the crystallite sizes [40]. It would be recalled that the dislocation motion constitutes one of the mechanisms for plastic deformation. Through the reduction in the dislocation slip by the high number of point defects, plastic deformation will be extremely hindered, which will culminate in the reduction in wear loss, and the increment in wear resistance [41]. Moreover, interstitial atoms, which are formed by a point defect, have the tendency of forming grid systems that are a perfect dislocation blocker, and subsequently, increase the wear resistance of HEAs. Hashimoto et al. [42] investigated the rate of vacancy mobility on CoCrFeNi-based high-entropy alloys. It was observed that the CoCrFeNiMn possessed a lower vacancy mobility energy than 316 stainless steel, while CoCrFeNiAl_0.3 possessed higher vacancy mobility energy.

These occurrences were attributed to the fact that the vacancy can be close to Co and Ni; but reacts with Al and Mn when they come in close contact. This is why the wear rate is higher in CoCrFeNiAl_0.3 than in CoCrFeNiMn as a high-vacancy mobility energy culminates into high plastic deformation. It was concluded that cobalt, nickel, manganese, and aluminum atoms influence the vacancy mobility in HEAs, inducing the slow-down in the formation of the microstructure and the enhancement in the precipitation of useful intermetallic phases. In another study [43], it was disclosed that, in an Al-rich environment, Al in AlNbTiZr can harmonize the single vacancy (react) and prevent its migration and diffusion to form large-size vacancy clusters. Therefore, the wear resistance can be enhanced by manipulating the volume fraction and dispersion of Al in AlNbTiZr.

2.3. Generation of Tribo-Films

SPSed HEAs have the capacity to generate tribo-films under certain loading and operational conditions. Tribo-film is a tinny layer of material which can develop from a material during a motion over a surface it is in contact with. One of the principal functions of the tribo-layer is the reduction in friction and wear; it also shields surfaces from damage. In comparison with conventional alloys, the HEAs form more superior tribo-films that act like a lubricant. This is made possible by producing oxide layers that form a protective coat on the surfaces in contact with another object. The consequences of HEAs’ structural disorder accompanying their formation can result in the evolution of carbide, iron, chromium, nickel, or niobium oxide films, depending on the elements in the alloy, which is a good solid lubricant. Vo et al. [44] synthesized the Co-free self-lubricating Al_0.8CrFeNi_2.2-Ag eutectic HEA. The wear test was conducted over a temperature range of ambient–900 °C. In comparison with Al_0.8CrFeNi_2.2, Al_0.8CrFeNi_2.2-Ag exhibited an excellent improvement in tribological properties. It displayed a lower coefficient of friction, and the COF reduced progressively as the sliding temperature increased. The COF was very stable across the whole test temperatures and was just slightly affected by the temperature. Al_0.8CrFeNi_2.2-Ag equally exhibited a lower wear rate at temperatures below 600 °C. At a temperature above 600 °C, the Al_0.8CrFeNi_2.2 displayed a lower wear rate and became more stable. At temperatures below 600 °C, the Cr₂O₃ and NiCr₂O₄ films were the prevailing oxides existing on the worn surface which acted as the tribo-layers for the reduction in the wear rate; and the NiCr₂O₄ inorganic acid salt has been reported by another author [45] as very important in decreasing the COF of materials at high temperatures. However, at temperatures above 600 °C, the Fe₂O₃ and Fe₃O₄ were the dominant tribo-films that aided in the reduction in COF and the improvement in wear resistance. Through the electrochemical boronizing of self-lubricating AlCoCrFeNi_2.1 HEA, the authors observed an evolution of a H₃BO₃ tribo-film on the contacting surface, which efficiently reduced the COF and wear by 25% and 80%, respectively [46]. So, all in all, the generation of tribo-films is one of the ways that HEAs resist wear and friction.

2.4. Evolution of Hard and Nanocrystalline Phases

The evolution of hard, nanocrystalline, and ceramic-like phases and precipitates on the surface rubbing against another object is another characteristic of SPSed HEAs in terms of improving tribological properties. The formation of the hard phase is induced either by the increased temperature or speed of the moving surfaces. Ideally, the hard precipitates that evolve during HEA formation or at high temperature during service include chromium carbides [47,48], nickel carbides/oxides [49], and chromium-rich B2 phases. Chromium carbides are very good at enhancing the wear resistance since they are very hard and resistant to chemicals. Nickel carbides are equally effective, but they are not as resistant to chemicals as Cr₃C₂. More so, B2 phase is a metastable phase which is rich in chromium and very good at improving tribology. Yu et al. [47] investigated the tribology and microstructure of chrome carbide-reinforced CoCrFeMnNi HEA using the discharge plasma-sintering process. It was observed that, with the evolution of the chromium carbide phase, the hardness increased by 1.5 times when 1.2 wt.% C was added to the HEA. When the percentage weight of C in the HEA was still small, the wear rate increased. But when the percentage weight of C increased to 1.2 wt.%, the wear rate decreased to a very low value of 1.59 × 10⁻⁴ mm³/Nm. The contact fatigue, adhesive, and oxidative wears manifested as the active wear mechanisms in the material. In another study investigating the precipitation of hard particles in HEAs, the hardness rose to 702 HV while the densification improved to 91% as the heating temperature was elevated to 900 °C, which was the optimum sintering temperature. The improvement in the hardness was as a result of the evolution of Al₂Cu₃ and Al₃Ni₅ intermetallic compounds together with the presence of face-centered cubic and body-centered cubic structures. For wear properties, the wear resistance increased more and the COF decreased more at 400 °C than at room temperature because of the evolution of an oxide layer on the sliding surface [49]. Moreover, it was reported that dispersing the dispersoids into the HEAs of solid lubricants, like oxides, nitrides, carbides, BaF₂/CaF₂, Ag, graphene, and MoS₂ can radically enhance their tribological properties in the dry environment and across all temperature ranges. The CaF₂ and graphene are the best at elevated temperatures, while the Ag and MoS₂ are ideal for the medium enhancement of wear resistance [50]. Hard particles play their own pivotal roles in decreasing friction and improving wear resistance.

2.5. Formation of Self-Healing Property

HEAs were reported as being active in self-healing, which implies that they would pull through from any mutilation and repeatedly retain their efficiency [51,52]. This is made possible through their capacity to form a complex microstructure with a high density of defects which leads to high hardness, and consequently, a high resistance to wear. The self-healing characteristic of HEAs occurs via ‘dynamic recrystallization’. When HEAs are subjected to wear-prone stress, the grain boundaries shift and rearrange in response to the stimuli which can culminate into the evolution of new grains possessing higher hardness and wear resistance. Dynamic recrystallization occurs when the material is at service, ensuring the effectiveness and durability of HEAs in critical applications, like in the bearings of turbines and compressors. One other self-healing attributes of HEAs is as a result of high atomic-level strains which come from the blending of elements with various atomic sizes. A simulation result showed high atomic-level strains, which disrupt the solid solution formation and induce the amorphization of alloys. Thermal spikes resulting from the heating up of grain particles usually generate localized melting and recrystallization, that bring the uniformity of the alloy with the reduction in the defect density [53,54].

2.6. Evolution of High Lattice Distortion

High lattice strain and distortion contribute to high crack resistance and consequently, high wear resistance and low COF. It worth noting that HEAs consist of a lot of constituent elements which possess dissimilar atomic radii that metamorphose into a huge atomic size mismatch, which induces the critical lattice distortion as the resulting dislocations interlock [55] as well as the ultimate wear resistance. In a study that seeks to determine the impact of Al addition on the microstructural, mechanical, and wear characteristics of TiZrNbHf refractory HEAs, the authors opined that the incorporation of Al provided the extraordinary hardening of the alloy and improved the wear-resistant property of the HEA, like the wear volume (Figure 6a). They attributed the improvement to a huge dissimilarity existing amongst the atomic radii of Al and other component elements in the HEA, and the high mismatch of the atomic size contributed to the high lattice distortion that elevated the slip resistance to dislocations [56]. In another study to determine the impact of WC on the wear characteristics of CoCrFeNi HEA, the authors gathered that, when the volume fraction of WC was increased, the diminishing of the sizes of the grain particles at the initial stage occurred, and then up-surged, stimulating an increase and then a decrease in the lattice constant. Due to the changes observed in the lattice constant, it was concluded that the incorporation of WC resulted in undissolved particles and intermetallics, together with a fraction of WC that entered the face-centered cubic structure.

This then formed the solid solution and evoked the lattice distortion of CoCrFeNi-WC HEA, which subsequently induced the solid solution strengthening and increased the wear resistance (Figure 6b) [57]. Wu et al. [58] investigated the effect of Mo addition on the lattice distortion and the subsequent strengthening of AlCrFeNiTi_0.5 HEA. It was observed that the lattice distortion initiated by the Ti in the B2 phase of the alloy led to solid solution hardening, and enlarged the lattice mismatch among the orderly B2 and disorderly BCC phase. So, as molybdenum was incorporated into the disorderly BCC phase, it enlarged its lattice cell size. Hence, the lattice mismatch existing among the disorderly BCC phase and the orderly B2 phase was reduced; and the disorderly BCC phase was strengthened. So, it can be said that the hardened disorderly BCC phase aided in increasing the global hardness/wear of the alloy. By and large, the fundamental basics behind inducing the solid solution strengthening through lattice distortion is by introducing the interstitial or substitution atoms into the HEA matrix. Then, the dissimilarity between the atomic radii of the matrix elements and the inclusion will evoke lattice distortion and lattice mismatch, which will generate solid solution strengthening [59].

2.7. Evolution of BCC Phase

Evolution of the BCC crystal structure in HEAs has been upheld to be a contributing factor to the enhancement of the friction and wear characteristics of high-entropy alloys. Among the crystal structures of HEAs, the BCC structure has a small atomic packing density of 68%, resistant to intra-crystalline dislocation, with profound strength, and hence, high-wear resistance [60]. To ensure the formation of the BCC lattice in HEAs, some elements must be incorporated, like Fe, V, Nb, W, Mo, Ta, and Cr [60,61]. Predominantly, the BCC phase is good at enhancing the hardness and wear resistance in HEAs [20,62]. Fu et al. [63] researched on the impact of Mo addition to the tribology of the CoCr₂-FeNiMo_x high-entropy alloy_. With the increase in Mo content, the σ-CrMo BCC crystal evolved in an FCC matrix. The hardness and tribology results showed that the hardness increased by 2.5 times, the COF decreased by 0.2 times, and the wear rate also decreased by 73% in comparison with the unreinforced HEA. Malatji et al. [64] incorporated W into AlCrFeNiCu HEA and observed that the W-doped HEA possessed higher hardness and wear resistance (lower wear loss and COF) than the undoped alloy. The improvements were attributed to the stabilization of the BCC phase by W doping. Feng et al. [65] developed AlCrFeNiV HEA using instantaneous plasma sintering and tested its microstructure and tribology. The microstructural characterization showed that the dual phases of BCC1 (aluminum, chromium, iron, and nickel) and BCC2 (vanadium and chromium) evolved. The high hardness of 1076 ± 15 HV and the low wear rate of 17.2 × 10⁻⁵ mm³/nm at dry mode were recorded. These improvements were attributed to the mixed BCC-induced solid solution strengthening. So, as the BCC structure is akin to high-strength and hardness in crystalline materials, it will be logical to state, and it has been confirmed via research, that the evolution of the BCC structure in HEAs will translate to a high wear resistance, low COF, and low wear loss.

2.8. Formation of FCC Phase

The FCC lattice structure is reputed for improving the ductility of HEAs; however, some of the FCC-inducing elements like Ni, Ag, Cu, and Al have some attributes in terms of improving wear resistance [10,66]. Yang et al. [67] investigated the hardness and tribology of CoCrFeNi and (CoCrFeNi)₉₀Ag₁₀ HEAs. It was observed that the hardness was reduced by 14%, the coefficient of friction reduced by 47%, and the wear rate was reduced by 91% when Ag was added to the original HEA. Even though the addition of Ag reduced the hardness, it improved the wear properties by the following mechanism: Recall that the enthalpy of the mixing (ΔHmix) of Ag, just like other elements mentioned (Ni, Cu, and Al) is positive so that Ag would not induce the formation of the solid solution in CoCrFeNi; rather, it induced the formation of spherical precipitates in the alloy. So, the silver and the solid solution that evolved developed into an interwoven nano-lamellar configuration, which gave the alloy a good self-lubricating characteristic [68]. In another study on the efficacy of Cu in terms of reducing the wear loss of HEAs, Verma et al. [69] discovered that, at elevated temperatures, Cu possesses a self-lubricating property. This was evidenced in CoCrFeNiCu_x HEAs. There was Cu segregation at the grain boundaries which enhanced the wear resistance, and the resistance increased when the Cu content increased. At a temperature of 600 °C, Cu-reinforced HEA was less wear-resistant than the unreinforced HEA under ambient conditions, which confirmed the self-lubricating property of Cu. Ye et al. [70] investigated the influence of Al inclusion on the tribology of CoCrFeMnNiAl_x HEA. They discovered that the hardness and lubricating ability of the HEA-Al increased richly, hugely decreasing the wear volume and coefficient of friction. The improvement was as a result of the formation of Al₂O₃ solid-lubricant film. The improvement in the high-temperature wear resistance of Al-reinforced CoCrFeMnNiAlx HEA was equally corroborated by other scholars [71].

2.9. Evolution of HCP Phase

Hexagonal close-packed (HCP) elements, like Co, Zr, Ti, Hf, Re, and Sc, incorporated into HEAs induce the enhancement of properties like elevated temperature oxidation resistance, strength, and plasticity [72,73]. Zhao et al. [74] fabricated AlNbTaZr_x HEA to analyze the influence of the Zr volume fraction on the microstructural properties and tribology of the HEA. The increase in the Zr content stimulated the enhancement of the strength and tribology of the alloy. Two wear modes were observed, abrasive wear, and brittle spallation mode. The former was obtained at a lower Zr content while the latter occurred at a high Zr content. So, the addition of Zr, which is an HCP element, actually improved the wear resistance. Pole et al. [75] investigated the tribology of TaTiVZrW and TaTiVZrHf HEAs at elevated temperatures. It will be noticed that W possesses a higher melting temperature than Hf (3422 °C against 1960 °C); hence, TaTiVZrW exhibited a higher hardness and resistance to elevated temperature tempering. It was observed that, at 423 K, TaTiVZrW had a higher wear resistance than TaTiVZrHf. However, at 723 K, the oxidation resistance of TaTiVZrW was weakened, and was heavily affected by a high-temperature oxidation wear. But, TaTiVZrHf HEA formed a mesomorphic eutectic lattice configuration which reduced the wear rate at an elevated temperature. The incorporation of Co and Ti into HEAs induced the formation of thick oxide layers that were impenetrable by oxygen atoms, and so, aided in improving their wear resistance [76,77]. Generally, the presence of the HCP phase in HEAs prevents the evolution of oxide intermetallics by obstructing the diffusion of oxygen into the alloy. This oxide would have been the particles that are easily removed through abrasion in the material. So, the absence of the abrasive oxide particles translates into better tribology. Moreover, the HCP phase increases the load-carrying capacity, strength, and solid lubrication of HEAs. Through these modes, the tribology of HEAs is improved by the HCP phase [78,79].

2.10. Incorporation of Non-Metal Elements

Incorporation of non-metals, like C, N, Si, and B into HEAs has been proven to be an essential procedure of enhancing their strength, hardness, and wear resistance by gap hardening and hard precipitate strengthening [60,80,81]. Recently, it was reported that B can be used to pin-up grain boundaries, and enhance the hardness and modulus of high-entropy alloys [60]. Boride precipitates are obtained when excess boron is incorporated into HEAs. The presence of boride precipitates enhances the wear resistance perfectly but decreases the plasticity of the HEAs [82]. The incorporation of carbon, on the other hand, stimulates the formation of metal carbides which enhances the strength and tribology of HEAs. Nevertheless, metal carbides are prone to pore formation and so reduce the corrosion resistance in HEAs [83,84]. It was reported that the appropriate C content in (TiZrNb)₁₄AlMo HEA not only improved the wear resistance but also enhanced the oxidation resistance [83]. The authors observed the evolution of the hard carbide particles with the HCP-α phase precipitate in a BCC matrix as a result of the incorporation of C into the alloy. These improvements in the wear and oxidation properties were attributed to the gap-strengthening and hard-phase-strengthening induced by the C inclusion but the corrosion rate increased due to the evolution of microcells in the HEA. The incorporation of Si in HEAs induces a heavy lattice distortion effect due to the fact that the atomic size of Si is very large, and larger than most non-metals. So, the intense lattice distortion evoked by the Si addition increases the wear resistance via gap strengthening [85]. Meanwhile, Si addition does not only increase the wear resistance but induces grain refinement in the microstructure. More so, silicide has the propensity of enhancing the elevated temperature hardness and oxidation resistance properties of high-entropy alloys [86]. In a study to determine the impact of Si addition on the mechanical and wear properties of Al_0.2Co_1.5CrFeNi_1.5Ti_0.5 HEA, Xin et al. [87] observed that the incorporation of Si decreased the stacking fault energy of high-entropy alloys, thereby enhancing the strength and wear characteristics of the HEA. Studies have shown that nitride-HEAs possess a very high hardness and exceptional wear resistance [88,89]. Si et al. [90] studied the strength and wear characteristics of the TiVCrZrWNx high-entropy alloy. As the quantity of N was increased, the hardness of the alloy rose by 5.6 GPa, and the COF reduced by 0.2 with a decline in the corrosion current density. This confirms that the addition of N to HEAs increases the hardness, wear, and corrosion resistance. The greater confirmation of the exploits of N was observed in another experiment where N was introduced into (CuNiTiNbCr)Nx HEA. When the quantity of N was increased, the configuration of the high entropy films migrated from only the amorphous structure to an amorphous-nanocrystalline nanocomposite configuration with subsequent increment in the hardness, toughness, and wear resistance [91].

In summary, the incorporation of elements that induce the formation of the BCC phase, like Fe, V, Nb, W, Mo, Ta, and Cr in HEAs, helps in improving the wear resistance of the alloy. Also, introducing the FCC-forming elements like Ni, Ag, Cu, and Al goes a long way in terms of enhancing the wear properties of HEAs. Then, again, the incorporation of non-metals, like C, N, Si, and B into HEAs has been proven to be an essential procedure of enhancing their strength, hardness, and wear via gap hardening and hard precipitate strengthening. These elements, when introduced in an optimized manner, can enhance not only the tribology of the alloy but also the mechanical and thermal properties.

3. Tribological Behaviors of HEAs Prepared with SPS

As stated earlier, one of the characteristics of HEAs is their exceptional tribological properties which have endeared them to be utilized in a plethora of critical applications. The high-entropy effect, lattice distortion, sluggish diffusion, cocktail, and other effects incumbent on HEAs are among the intrinsic characteristics leading to the improvement in their tribology. High wear resistance, low wear loss, and low COF are readily exhibited by HEAs because of some of the following factors: precipitation of hard particles, evolution of BCC phase or combination of BCC + FCC, evolution of tribo-films, etc.

Table 3. Tribological characteristics of SPSed high-entropy alloys.

SPSed HEAs	Sintering Parameters	Properties	Remarks	Ref.
FeCoCrNiMoSi0.5	ST = 1150 °C,	COF = 0.369 WR = 0.0000292 mm3/Nm,	Phases: FCC (Fe), FCC(Mo), Si intermetallic. Improvement: the addition of 0.5 wt.% Si induced abrasion wear that is less severe than adhesion and oxidation.	[92]
AlCrFeNiTi	ST = 950 °C, DT = 8 min, SP = 40 MPa, HR = 100 °C/min	COF = 0.3, WR = 2.66 × 10−6 mm3/Nm (@5 N) and 5.06 × 10−6 mm3/Nm (@15 N)	Phases: BCC1 (AlNi2Ti) and BCC2(CrFe). Improvement: evolution of fine grains leading to grain boundary strengthening.	[93]
Ti-Al-Mo-Si-Ni	ST = 800–1000 °C (1000 °C was the best), DT = 8 min, SP = 50 MPa	COF = 0.23	Phases: BCC, TiSi2, and Ni2Si2 intermetallic compounds. Improvement: incorporated Ni and Mo which induced high hardness.	[94]
CuMoTaWV	ST = 1400 °C, DT = 10 min, SP = 40 MPa, HR = 50 °C/min	COF@RT = 0.45, COF@600 °C = 0.54 WR@RT = 4 × 10−3 mm3/Nm, WR@600 °C = 4.5 × 10−2 mm3/Nm	Phases: BCC + V-rich precipitates., Improvement: formation of self-lubricating V-rich phase (V2O5), W, and Ta tribo-films.	[95]
CoCrFeNiMn-10Cr3C2	ST = 1050 °C, DT = 20 min, SP = 30 MPa, HR = 50 °C/min	COF@RT = 0.43, COF@800 °C = 0.35, WR@RT = 0.980 × 10−5 mm3/Nm, WR@800 °C = 7.17 × 10−6 mm3/Nm	Phases: FCC + Cr7C3. Improvement: as a result of the hardening influence of the Cr7C3 phase and other oxides generated at elevated temperatures.	[96]
AlCoCrFeNi2.1	ST = 800–1200 °C (1000 °C was best)	COF = 0.19, WR = 0.23 × 10−3 mm3/Nm	Phases: BCC + FCC. Improvement: BCC + FCC strengthening that prevented dislocation.	[97]
FeNiCo15Cr5	ST = 1100 °C, DT = 8 min, SP = 50 MPa, HR = 55 °C/min	COF = 0.33, WR = 4.0 × 10−5 mm3/Nm	Phases: FCC (γ) + Cr. Improvement: Co oxides induced the formation of a lubricating glaze layer.	[98]
CoCrFeNiMo0.3	-	COF@5 N = 0.711, COF@50 N = 0.596 WR@5 N = 0.59 × 10−3 mm3/Nm, WR@50 N = 0.42 × 10−3 mm/Nm	Phase: FCC. Improvement: incorporation of Mo enhanced the strength and tribology.	[99]
CoCuFeMnNi	ST = 750–950 °C, DT = 8 min, SP = 40 MPa, HR = 140 °C/min	COF@RT, 5 N = 0.65, 10 N = 0.5; COF@600 °C, 5 N = 0.5, 10 N = 0.5; WV@RT, 5 N = 2 mg, 10 N = 4 mg; WV@600 °C, 5 N = 10 mg, 10 N = 19 mg.	Phase: FCC. Improvement: evolution of oxide tribo-films on the wear tracks at 600 °C resulted in low wear volume and low COF.	[100]
FeCoNiCuAl-30TiC	ST = 1000 °C, DT = 10 min, SP = 30 MPa	COF = 0.35, WR = 0.1 × 10−4 mm3/Nm	Phases: BCC + FCC. Improvement: precipitation and fine grain strengthening. Formation of oxide layer at high temperature.	[101]
NiCoCrFeZr0.4	ST = 900 °C, DT = 9 min, SP = 45 MPa	COF = 0.7, WR = 0.001 × 10−3 mg/m	Phase: FCC. Improvement: incorporation of Zr caused the enormous decrease in coefficient of friction.	[102]
AlCoCrFeNi	ST = 1000 °C, DT = 5 min, SP = 30 MPa	WR = 2.61 mm3/Nm	Phases: BCC + FCC. Improvement: the effect of BCC strengthening.	[103]
AlCrCuMnNi	ST = 800–900 °C, DT = 5 min, SP = 40 MPa	COF@RT = 0.425, @400 °C = 0.35,	Phases: FCC + BCC. Improvement: higher resistance at 400 °C because of the evolution of a strong oxide layer on the contacting surface with reduced COF.	[49]
CoCrFeNi	ST = 1100 °C, DT = 5 min, SP = 35 MPa, HR = 100 °C/min	COF@RT in vacuum = 0.68, @800 °C = 0.48, WR@RT in vacuum = 8 × 10−4 mm3/Nm, WR@800 °C = 1.3–8.0 × 10−4 mm3/Nm	Phases: FCC. Improvement: plenty of oxides in air usually formed tribo-layer which lowers wear rate more than what is obtainable in a vacuum.	[104]
CoCrFeMnNi	ST = 1173 K, DT = 10 min, SP = 50 MPa	COF = 0.66,	Phases: FCC + CrN intermetallic. Improvement: plasma nitriding of SPSed HEA reduced the coefficient of friction and wear volume.	[105]
Al0.25CrTiMo	ST = 1400 °C, DT = 20 min, SP = 30 MPa, HR = 60 °C/min	COF@RT = 0.78, @800 °C = 0.35; WR@RT = 90 × 10−5 mm3/Nm, WR@800 °C = 0.02 mm3/Nm	Phases: BCC. Improvement: the formation of Cr and Mo oxides provided effective lubrication, reducing the wear rate.	[106]
CoCrFeMnNiC0.6	ST = 1200 °C, DT = 8 min, SP = 30 MPa, HR = 100 °C/min	COF = 0.68; WR = 0.47 × 10−5 mm3/Nm	Phases: FCC + M7C3. Improvement: as a result of the crystallization of strong M7C3 carbides with minimal pores.	[80]
CoCrFeMnNi-2CNTs	ST = 1273 K DT = 5 min SP = 55 MPa HR = 100 K/min	COF = 0.66 WR = 0.85 mm3/Nm	Phases: FCC + M7C3. Improvement: the presence of Cr in the FCC phase had an outstanding influence protection against oxidative wear of the HEA.	[107]

Keys: ST = sintering temperature; DT = dwell time; SP = sintering pressure; HR = heating rate; WR = wear rate; RT = room temperature; WV = wear volume/mass.

summarizes some of the HEAs prepared by SPS and their tribological properties and production parameters. It can be seen from Table 1 that the BCC phase has a lower COF than the FCC phase. This could be the result of high strength and hardness exhibited by the BCC phase.

4. Wear Mechanisms in HEAs

There exist five principal modes of wear, namely oxidative, corrosive, adhesive (delamination), fatigue, and abrasive wear modes. Oxidative wear is generally encountered in high-temperature applications, where the presence of oxygen and high temperatures induce the swift deterioration of materials by removing wear protective films. Corrosive wear occurs when there is the chemically induced deterioration of a material, which can engender its wear and failure. The adhesive wear mode occurs when two surfaces are glued together and slit when they are unglued, triggering wear on the two surfaces. Fatigue wear occurs via the deterioration of a material as a result of recurrent loading that triggers the collapse and wear of the part of the material that is loaded over time. Abrasive wear takes place when two contacting objects brush over one another, causing one of the objects to lose some particles of the surface.

4.1. Oxidative Wear

When oxygen atoms react with the atoms of the HEAs, especially at high temperature, they break and displace them from the surface. The principal dynamics that instigate oxidative wear include the presence of oxygen or oxidizing agents like O₃, HNO₃, H₂O₂, and Cl; high temperature; and moisture. Meanwhile, HEAs are premeditated to be corrosion-resistant; however, they can sometimes be vulnerable to oxidative wear. This is because HEAs usually contain a combination of dissimilar elements, some of which are susceptible to oxidation. Take, for instance, Cu, which is a regular element contained in HEAs, but which is very prone to oxidation. If it oxidizes, it creates pits and cracks on the material surface, which is a nucleation ground for fast-tracked wear [108]. Most often, oxidative wear ensues at elevated temperatures, at points where oxide layers develop on the periphery of the HEA, and then become detached by an opposing contacting body, whereas the newly formed exterior is oxidized again and again; thus, the reaction continues [104,109]. The oxidation reaction accelerates the wearing off of the material surfaces because the formed oxide films develop a weak configuration at high temperatures due to the elevated temperature tempering. The addition of some elements, like W, Hf, and Al, aids in suppressing oxidation at elevated temperatures in high-entropy alloys. In the research by Pole et al. [75], it was observed that, at a medium temperature of 423 K, the presence of W in TaTiVZrW aided the high-temperature oxidation resistance more than what Hf could contribute in TaTiVZrHf. However, at an elevated temperature of 723 K, the oxidation suppression of TaTiVZrW was weakened, and was heavily affected by high-temperature oxidation wear. But TaTiVZrHf HEA formed a mesomorphic eutectic lattice configuration which improved the oxidative wear resistance at that high temperature. In another study, Wu et al. [110] discovered that raising the volume fraction of Al in Al_xCoCrCuFeNi HEA induced a high resistance to oxidative wear. There was an Al₂O₃ film that was generated when the bodies were rubbing against each other, and this aided in greasing the surface of the HEA and in reducing their asperity, COF, and wear rate. Jin et al. [111] worked on the COF of FeNiCoAlCu HEA. It was observed that the COF value in the temperature range of 20–400 °C was 0.9, but when increased to 600–800 °C, it reduced to 0.3. The X-ray photoelectron spectroscopy analysis disclosed that the generated oxide film was composed of Fe₂O₃, Fe₃O₄, Al₂O₃, and CuO, and that the repeated fracture and regeneration of oxide films contributed to the decrease in oxidative wear at the elevated temperature. To further demonstrate the function of Al in reducing the oxidative wear of HEAs, Ye et al. [70] discovered that an oxide film consists of three layers, as shown in Figure 7, when microstructural analysis was conducted on Al_xCoCrFeMnNi HEA. In the absence of Al in the alloy, the oxidation wear was very severe. But, when Al was introduced, a dense aluminum oxide film evolved, and there was a decrease in the size of the oxide layer which implied that the aluminum enhanced the oxidation resistance. As the content of Al increased, the manganese fraction in the oxide film kept decreasing. For instance, in Figure 7e, at point 13, the aluminum fraction was 29.49 at%, while the manganese fraction was 27.08 at%; at point 15, the aluminum fraction increased to 33.42 at%, while the manganese fraction decreased to 4.76 at%. The explanation is that the evolved dense aluminum oxide at the instance of oxidation obstructed the penetration of manganese ions into the inner portion of the HEA; thus, this enhanced the overall oxidation suppression of the HEA.

In contrast to the popular opinion regarding the impact of aluminum in suppressing the elevated temperature oxidation of HEAs, it was observed that the MoTaTiCr medium entropy alloy performed better than MoTaTiCrAl HEA. Here, MoTaTiCr generated a CrTaO₄ film which protected the alloy from oxidation. The oxide film aided in preventing the volatilization of Mo which is the major factor that gives the MoTaTiCr antioxidant property at an elevated temperature [112].

4.2. Corrosion Wear

Corrosion wear can be described as an electrochemical process which can take place during friction and wear [82,86,94]. Corrosive media include acid, base, and salt; where the 3.5 wt% sodium chloride salt solution is usually employed to mimic the marine environments. The application of HEAs in these corrosive environments exposes them to corrosion wear. However, the prevalent effects like sluggish diffusion or cocktail effects of HEAs stimulate the evolution of a highly disordered, metastable microstructure which comprises an aggregation of phases, like amorphous phases, semi-crystalline phases, and fully crystalline phases, which prevent the ravage of corrosion wear. The randomness in the microstructure further evokes the formation of complex chemical moieties and ions, interlocked grain boundaries, and refined grains which synergize to improve the corrosion wear resistance [113]. Zhang et al. [114] developed the (CrNbTiAlV)N_x HEAs film to study its corrosion wear characteristic. It was observed that a refined microstructure efficiently obstructed the penetration of chloride ions into the alloy, showing strong improvement in the corrosion wear resistance. Generally, the corrosion wear takes place when there is a sliding movement of one body over another in a corrosive medium. Normally, when a material is at rest or when it is not gliding over another, the passive film readily forms, which prevents the material from further electrochemical degradation. But, when gliding resumes, there is a gradual wearing away of the protective layer, exposing the surface to the corrosive environment. So, the combination of wear and corrosion in a surface undergoing a sliding motion is termed corrosion wear. Here, the environment must be corrosive and there must be a sliding motion of one body over another. Chen et al. [115] studied the corrosion wear behavior of Al_0.6CoCrFeNi HEA under various media. In Figure 8, the results obtained were presented. In ambient air (Figure 8a), there were the cracks and heavy delamination of particles. The material pull-off was parallel to the direction of corrosion wear. In Figure 8b (corrosion wear under acid rain), there was a formation of oxide films. Peeling pits, grooves, and delamination were observed in the worn out surface. However, the material pull-off was lighter here than in the ambient air because acid water acted as coolants and lubricant in reducing the wear. In Figure 8c (HEA in sea water), there was a greater delamination of the materials than in the acid rain. This was because the sea water contained chloride ions which attacked the oxide film and induced its dissolution. There were big grooves as were obtained in the ambient air. Hence, the presence of Cl^- ions aggravated the corrosion wear in the alloy immersed in sea water.

4.3. Adhesive Wear

Adhesive wear is experienced in HEAs when the applied load supersedes the ultimate yield stress of the body and evokes plastic deformation, conveying the worn out material to the tribo-pair [116]. Conditions that are favorable to adhesive wear include excessive load, weak coating, high room temperature, or the high joint dissolution of both the tribo-pair and contacting surface. Adhesive wear is exhibited as plough furrows on the worn surfaces [117,118]. It has been revealed that the adhesive wear mode is more catastrophic than the abrasion wear mode because they generate a more severe COF and wear level [116]. If the bond energy and severing depth of a material are low, its exposed area would develop an unbroken greasing film which decreases the COF.

Yang et al. [92] developed FeCoCrNiMoSix HEA via SPS and characterized its wear behavior. The volume fractions of Si incorporated included 0.5, 1.0, and 1.5. The results obtained revealed that, when the percentage weight of Si increased, the wear mode transited from abrasive wear to the delamination (adhesive) wear mode, as shown in Figure 9, followed by grain coarsening. There was a higher percentage of delamination wear which stimulated the higher wear rate in FeCoCrNiMoSi_1.0 and FeCoCrNiMoSi_1.5 than in FeCoCrNiMoSi_0.5. So, adhesive wear ploughs off more materials than abrasive wear. Even though the addition of the Si element improved the strength of the HEA, it equally raised the wear level (Figure 9d) and pull-off of materials from the surface of the alloy. It increased the roughness and grains of the surface (Figure 9b,c). The introduction of solid lubricants like graphite and molybdenum sulfide into HEAs can ameliorate adhesive wear.

4.4. Fatigue Wear

Fatigue wear occurs as a result of the cyclic loading and unloading of a body with weight that exceeds its fatigue strength and manifests as cracks or spalling damage on the contacting surface; sometimes, it may result in pitting corrosion [119]. Cai et al. [120] investigated the effect of the addition of titanium carbide to the FeMnCrNiCo HEA coating. The authors observed the refinement of grains and improved dislocation energy, which increased the wear suppression of HEA surfaces. The addition of excess TiC induced high brittleness in the alloy with localized micro-cracks identified as fatigue wear. Fatigue wear can be ameliorated by reducing the cycle of repeated loading and the addition of hard particles or coatings on the surfaces of HEAs.

Liu et al. [121] worked to improve the wear resistance of FeCoNiCrMn HEA by reinforcing it with carbides via in situ and ex situ methods using MA and SPS consolidation. The HEA-Cin situ was prepared by soaking carbides (M₂₃C₆ and M₇C₃) in the alloy, and this generated a yield strength of 1756 MPa and a maximum compressive strain of 10.28%, which was about 14.5% higher than that of most HEAs found in the open literature. However, the HEA-Cex situ was prepared by adding extra 5 wt% TiC/TaC, and this generated a yield strength of 1760 MPa and a maximum compressive strain of 7.85%. Meanwhile, the mean COF of ex situ C-reinforced HEA was 0.26, while that of the in situ C was 0.32. The microstructural analysis of the wear showed that the in situ C-reinforced HEA manifested adhesive and little oxidation wear, but ex situ C-reinforced HEA showed mostly showed oxidation wear with traces of adhesive and fatigue wear. So, the addition of carbides was an excellent way of improving the fatigue wear resistance in HEAs.

4.5. Abrasive Wear

Abrasive wear or micro-shear wear manifests as the mini-cut off of the materials from contacting tribo-pairs as a result of relative motions amongst them and culminating into parallel grooves on the worn surfaces [122]. The principal theory of abrasive wear is Archard’s formular. It states that “the volume of material removed is proportional to the normal load, the distance of sliding, and inversely with the hardness of the materials” [123].

Wvol = kFd/H (i)

where Wvol is the wear loss, k is the wear coefficient, F is the normal load, d is the sliding distance, and H is the hardness of material. So, to decrease the quantity of material removed through abrasion wear, it is imperative to decrease the applied load, the sliding distance, or increase the hardness of the material. To obtain low-abrasion wear, some processes like lubrication, surface hardening, and surface texturing are undertaken. Li et al. [124] developed the wear model for FeNiCrCoCu HEA using dynamics simulation. The result disclosed that, when the strength of the coating was high, the piercing level of the abrasion wear decreased, together with the tangential force. When the abrasive piercing speed was increased, there was the inducement of tempering of the substance which decreased the tangential force. The modeling result of the microstructure of the lattice configuration and dislocation showed that the worn surface generated ‘Hirth dislocation locks’, which prevented the initiation of dislocation and decreased the dislocation density and surface scrape-off.

4.6. Wear-Level Categories

The linear relation of Archard’s equation is true only to some given loading [24,125]. It does not hold very low loads [126] or extremely high loads [127]. So, wear occurrence is basically categorized into three levels, namely low level, mild level, and severe wear level categories. Based on this categorization, Aghababaei et al. [128] disclosed that, in the low-wear level, the surface roughness deforms predominantly through elasto-plasticity, and does not generate any debris. For the mild wear regime, a fraction of the touching surface asperities is pulled-off via micro-cracking, producing little wear debris. Transitioning from the low-wear regime to the mild-wear regime is a function of the applied load. Linear correlation among the load and wear loss in the mild wear level obeys the mathematical relation that the quantity of the wear debris removed is a function of the junction (contacting surface) area [129]. Severe wear is accompanied by large subsurface cracks, with the generation of debris particles that have particle sizes equal to that of the apparent contacting area. Hence, in this regime, the rectilinear correlation among the wear loss and the applied load is broken and this is the key indicator of severe wear.

5. Applications, Challenges, and Future Research on Tribology of HEAs Prepared by SPS

5.1. Potential Applications of HEAs Prepared by SPS

HEAs have excellent tribological properties. Consequently, they have manifold areas of prospective applications. Here, some key areas of applications are discussed.

(I) Automobile components: The wear resistance and strength without compromising the ductility of HEAs make these exceptional materials for the production of robust automobile components like engine valves, gears, brake calipers, shafts, connecting rods, engine pistons, and ball joints. Other properties displayed by HEAs that are requisites for the production of automobile components include resistance to fatigue load, as well as resistance to corrosion, wear, and impact loads. The AlCoCrFeNi HEA is light, has a high yield strength of 1263 MPa at 773 K, a high ultimate compressive strength (UCS) of 1702 MPa, and a plasticity of 19.9% up to the temperature of 773 K. This shows that it is suitable for use in structural and high-temperature applications like gears, engine pistons, and valves [130].

(II) Medical and pharmaceutical applications: Popescu et al. [131] developed TiZrNbTaFe HEA and characterized its properties for biomedical implants. This HEA was compared with the conventional implant, Ti6Al4V alloy. It was reported that corrosion heavily affects Ti64 because of the presence of α and β phases in the microstructure which initiate galvanic corrosion at their interfaces. But the novel HEA exhibited only the β phase, which is more resistant to corrosion. Moreover, the presence of Ta increased the resistance of the HEA to corrosion because it is very resistant to Cl⁻ by forming the Ta₂O₅ protective film. It was therefore concluded that TiZrNbTaFe HEA is more biocompatible than Ti6Al4V for biomedical applications. Two major ways by which HEAs are employed in the medical sector are in orthopedic implants (comprising hip and knee) and in dental implants. HEAs employed for hip and knee replacements usually possess high strength, low elasticity, and high biocompatibility; which are properties they share with natural human bones. CoCrFeMnNi HEA has been successfully applied in the replacement of hip and knee and has exhibited superior characteristics in clinical trials [132].

(III) Purification of water: HEAs have a high adsorption property as a result of their high surface area which provides a large area for the adsorption of contaminants. More so, the high degree of disorder in HEAs enables them to possess large binding sites for the water contaminants. The high disorder will not give the contaminants a safe haven to pile up and cause fouling in the water channel [133]; coupled with their refined microstructure. It was reported that CoCrFeMnNi HEA is a good anti-fouling agent in water purification. Furthermore, HEAs have a refined microstructure devoid of pores which does not give binding sites for water contaminants. Contaminants bond better on coarse and porous surfaces and wreck their havocs. It is equally worthy noting that HEAs have a high corrosion resistance and superhydrophobicity [134]. So, microorganisms with a high affinity to inhabiting corroded surfaces are dearth in surfaces cladded with HEAs because of the lack of corrosion ions or radicals.

(IV) Microjoining: HEAs can efficiently perform more than conventional alloys in joining small components with the application of pressure and heat [135]. This is because HEAs possess a higher strength and higher thermal properties with low weight. HEA joining is useful in the production of micro-electro-mechanical system (MEMS) devices like sensors and actuators together with repairing their small parts [136]. The high strength of HEAs is required to withstand the high stresses domiciled in a joint. The low weight is a requisite in today’s need for the miniaturization innovations of components and products, hence the need for the use of HEAs. Replacing conventional alloys with HEAs for high thermal applications is imperative because refractory HEAs can withstand high temperatures without losing strength coupled with a high creep resistance. FeCrAl-XY HEA (XY = Si, C, N, B), for instance, can be applied for elevated temperature joining as they can withstand temperatures of up to 1450 °C with appreciable oxidation and corrosion resistance [137].

(V) Space shuttle components: Aerospace components were traditionally built with superalloys and single crystal alloys. However, the narrative is changing since the discovery of HEAs. Due to their high wear resistance and strength at elevated temperatures, HEAs are being used to replace those conventional alloys employed in jet engine components like the turbine blades, compressors, and a combustor. For instance, the rotor of airplanes was hitherto built with ferritic steel. But, it has been disclosed that GE Aviation has developed NbMoTaW HEA which possesses better properties than ferritic steel [138]. Such excellent properties included being able to withstand temperatures above 800 °C, as well as possessing high strength and high corrosion resistance at elevated temperatures with sound creep resistance.

5.2. Tribological Challenges and Further Work on HEAs Prepared by SPS

It is worth clarifying that HEAs prepared by SPS are challenged by oxide inclusions during the sintering operation. The trapped oxides are most often the site for stress concentration. Once such oxides become entrapped, the area becomes the point of crack nucleation and propagation when there is even a little dislocation slip [139]. In order to offset this challenge of entrapping oxides during the SPS of HEAs, the inertness of the sintering chamber should be beefed up so as to have zero interference of O₂ and other oxidizing agents. Moreover, there is the need to introduce de-oxidizing agents into the sintering chamber or the HEA powders being sintered, which would barricade any incursion of oxide inclusion.

Again, some HEAs are sintered at a very high temperature and pressure. This may likely introduce residual or thermal stress which may negatively affect the suppression of wear by the alloy [140]. To tackle the issue of residual stress, heat treatment after fabrication is recommended. It is called “post-processing heat treatment”. Here, the sintered sample is fired to a temperature below the melting temperature of the HEA and left at that temperature for a reasonable amount of time in order to stimulate the relaxation and redistribution of the stress within the material and ameliorate the stress. Moreover, it is advised and recommended that the sintering temperature and pressure of HEAs should be as low as possible to prevent the growth of the thermal and residual stresses. Although this may induce high porosity and low densification in the alloy, the optimization of the sintering parameters using low sintering values can enhance these properties.

Furthermore, some HEAs processed through the high temperature may be affected by the high-temperature softening of the material [141]. The softened materials are prone to aggressive friction and wear. Hence, it is recommended that the sintering of HEAs should be carried out with optimized low sintering parameters, especially at a very reduced dwell time and increased heating and cooling rates.

Another tribological challenge suffered by HEAs is the evolution of a tribo-film. Even though a tribo-film is good at improving the wear suppression capacity via the reduction in the COF, sometimes, it may pose some negative influences on wear behavior. Some negative influences of the tribo-film include micro-cracks, the delamination of stressed surfaces, etc. One method of ameliorating this challenge is through surface treatments. The surface improvement of HEAs via laser surface cladding [142] or shot peening would be able to generate a more homogenous microstructure on the SPSed surface of the HEAs. This practice can eradicate the evolution of tribo-films through the removal of the potions susceptible to tribo-film formation—and can improve the metallurgical bonding of the alloys through the evolution of refined and homogenous structure [143]. The second method of ameliorating the negative influences of tribo-films is using the surface coating on HEAs prepared by SPS. Surface coatings like diamond-like carbon (DLC) [144], titanium nitride (TiN) [145], and chromium nitride (CrN) [146] can be used as a shield to prevent friction and wear on HEAs. The coatings can be applied through chemical vapor deposition (CVD) or physical vapor deposition (PVD). These coatings are hard particles which can protect the surface of the HEAs by acting as a barrier in standing against abrasive and adhesive wears.

6. Conclusions and Recommendation

A review of the tribological properties of HEAs prepared by SPS has been undertaken and some salient points that can be filtered from the study are as follows:

• HEAs are endowed with a high wear suppression characteristic, low wear loss, and low COF, which is why they can be applied in the production of the landing gears of an airplane, automotive piston, clutch and braking system, turbine blades, and compressor parts. Their high tribological properties increase their durability and optimal functionality.
• Factors that equip HEAs with high tribology besides the four major effects prevalent in HEAs include the evolution of hard particles, BCC phase, FCC phase, HCP phase, refined grains/microstructure, tribo-films, point defects, self-healing property, and so on. So, HEAs are profoundly endowed to resist friction and wear.
• The incorporation of elements that induce the formation of the BCC phase, like Fe, V, Nb, W, Mo, Ta, and Cr in HEAs helps in improving the wear resistance of the alloy. Also, introducing the FCC-forming elements like Ni, Ag, Cu, and Al goes a long way into enhancing the wear properties of HEAs. Then, again, the incorporation of non- metals, like C, N, Si, and B into HEAs has been proven to be an essential procedure of enhancing the strength, hardness, and wear resistance of them via gap hardening and hard precipitate strengthening.
• Adhesive wear has been adjudged as a very destructive and more severe than abrasive wear because of the high COF that accompanies adhesive wear. The introduction of solid lubricants like graphite and molybdenum sulfide into HEAs can ameliorate adhesive wear. It is still the most probable wear mechanism that occurs in materials.
• Challenges militating against the optimal performance of HEAs prepared by SPS include oxide inclusion, thermal/residual stress, tribo-films, and high-temperature tempering. So, efforts to eradicate these challenges are encouraged.

The following recommendations are made:

i.

The inertness of the spark plasma sintering chamber should be beefed up so as to have zero interference in O₂ and other oxidizing agents.

ii.

Post-sintering treatment like tempering is recommended to partially mitigate the accumulation of residual stress.

iii.

It is equally advised to conduct the sintering at the lowest optimized parameters to avoid thermal stress.

iv.

The spark plasma sintered specimens of HEAs are recommended to be coated with hard particles like DLC, TiN, and CrN which can act like a barrier in preventing the abrasive and adhesive wears.

v.

The post-sintering surface treatment like laser cladding or shot peening is recommended as a procedure which will protect the surfaces of sintered HEAs from destructive tribo-films.